Qualia · about 4 minutes
The Synaesthesia Booth
For a few people in every hundred, Tuesday is green, the number seven is faintly sardonic, and the name Michael tastes of cold soup. Not "reminds them of" -- is. Involuntarily, consistently, since childhood.
Before you file that under other people's brains: you almost certainly do a small version of it, and we can prove it in about eight seconds.
One of these shapes is called bouba. The other is called kiki. Neither word means anything in any language. Which is kiki?
Now a harder one. Hidden in these digits, the 2s form a shape. Find it, then press the button -- we are timing you, but only so we can tell you something afterwards.
0.0s
Last question, and answer it quickly and honestly. Look at these:
Do any of them have a colour -- not a colour you'd choose for them, but one they already have?
An extra channel, in ordinary brains
Synaesthesia is the involuntary joining of senses or categories that are separate for most people: letters and numbers with colours (the commonest form), sounds with colours, days of the week with positions in space, words with tastes. Around four per cent of people have some form of it.
The three things that make it a real phenomenon rather than a vivid metaphor are worth knowing. It is involuntary -- you cannot decide that seven is now blue any more than you can decide this page is blue. It is consistent: test a synaesthete on which colour goes with which letter, come back two years later unannounced, and the answers match to a degree that non-synaesthetes cannot fake even when told to try and given a week to revise. And it runs in families.
The leading account is cross-activation between neighbouring cortical areas -- in grapheme-colour synaesthesia, the region that handles letterforms sits strikingly close to a colour area, and unusually persistent connectivity between them would produce roughly what synaesthetes report. Everyone starts life with far more cross-connection than they end up with; one story is that synaesthetes prune less.
And the shapes you just matched
Which is where bouba and kiki come in -- because if synaesthesia were a completely separate kind of brain, you would expect no trace of the same wiring in everyone else. Instead, roughly ninety-five per cent of people match those two nonsense words to those two shapes the same way, across languages, cultures, and in young children.
That is a cross-modal mapping -- sound to shape -- running in almost everybody, for no reason anyone chose. The mouth makes a sharp shape for kiki and a rounded one for bouba; the sound is jagged or smooth; and somewhere below the level of decision, your brain files those with the matching outline.
So the honest picture is not two kinds of people. It is a dial. Everybody has some cross-talk between senses. A few per cent have a great deal of it, vividly and consistently -- and the rest of us have just enough to be certain, without being able to say why, that the spiky one is called kiki.
Why this room is in a museum about consciousness
Two doors back, Exhibit № 10 raised the possibility that your inner experience of colour differs from everyone else's and can never be checked. Then the apple and the voice showed you differences that turned out to be real, common, and simply never asked about.
Synaesthesia is the strongest version of that argument, because here the extra experience is not subtle, not metaphorical, and not deniable -- it is a whole additional stream of qualia, present in millions of otherwise entirely ordinary people, and it was dismissed as fantasy or drug-talk for most of the twentieth century before anybody bothered to test whether it was consistent.
Which leaves the question the whole wing has been building toward. If a minority can experience a colour that nothing in the room is emitting -- and everybody else's brain is quietly doing a smaller version of the same trick -- then the assumption that any two people are having the same experience of anything is a convenience, not a finding.
Take it further
V.S. Ramachandran and Edward Hubbard's work in the early 2000s did more than anything to make this respectable, including the pop-out test you just did. The shape-sound effect goes back to Wolfgang Köhler in 1929, who used takete and baluba; bouba and kiki are the modern names. For first-person accounts, Patricia Lynne Duffy's Blue Cats and Chartreuse Kittens is the classic, and the overview article is unusually good.
One honest note about the pop-out test: the real version is carefully controlled for how much the digits differ in shape, and timing one person once on a laptop measures nothing. If you were fast, that is not evidence you are a synaesthete -- it is evidence you are good at finding twos. The genuine diagnostic is consistency over time, which needs a test today and another one in two years when you have forgotten this ever happened.